Human α 1 ‐microglobulin: Its measurement and clinical significance

α 1 ‐Microglobulin (α 1 ‐M), also called protein HC, is a low‐molecular‐weight (LMW) glycoprotein (about 30 kDa) with unique physicochemical properties. Using purified urinary α 1 ‐M as a standard and specific antibody against α 1 ‐M, an assay system for α 1 ‐M was developed, and the clinical significance of this protein was investigated by measuring total levels of α 1 ‐M under physiological and pathological conditions. α 1 ‐M is distributed in various body fluids: in serum, it consists mainly of free LMW α 1 ‐M and monomeric IgA–α 1 ‐M complex. The total α 1 ‐M level in serum and urine usually reflects LMW α 1 ‐M variation sensitively, and its determination is quite useful as an indicator of renal glomerulotubular dysfunction and hepatic dysfunction. Serum levels can vary, depending on IgA‐α 1 ‐M complex level, in parallel with the IgA concentration. The heterogeneity of α 1 ‐M purified from different sources of urine by different procedures and underestimation of IgA‐α 1 ‐M complex by solid‐phase antibody assays can be important causes for the discrepancy of serum levels between assays.